Radio Frequency Identification (RFID) systems are well known and are commonly utilized for item tracking, item identification, and inventory control in manufacturing, warehouse, and retail environments. For example, an RFID reader can scan for RFID tags associated with items in a controlled area, particularly for inventory control of the RFID-tagged items.
Briefly, an RFID system includes two primary components: a reader (also known as an interrogator), and a tag (also known as a transponder). The tag is typically a miniature device that can respond, via an air interface channel, to an RF interrogating signal generated by the reader. The tag is associated with an item to be monitored and can generate an RF responding signal in response to the RF interrogating signal emitted from the reader. The RF responding signal is modulated in a manner that conveys identification data (also known as a payload) back to the reader. The identification data can then be stored, processed, displayed, or transmitted by the reader as needed. One or more readers can be mounted in a controlled inventory area, for example, in an overhead location on the ceiling, and the readers can cooperate to locate any particular tagged item in the inventory area, for instance, by triangulation.
For superior RFID tag detection and locationing coverage, it is known to provide beamforming for individual readers with an array of antenna elements that transmit the RF interrogating signal through a transmit beam that is electronically steered and scanned both in azimuth, e.g., over a steering angle of 360 degrees around a vertical plumb line or vertical axis originating from the center of an antenna of a ceiling-mounted RFID reader, and in elevation, e.g., over a steering angle span of about 90 degrees angularly away from the plumb line, and that receive the return RF responding signal through a receive beam from the tags. These conventional antenna arrays typically feature identical individual antenna elements placed at regular intervals according to a predefined lattice across a supporting planar or conical surface,
Effective beamformed RFID reader scanning performance benefits from a relatively high port isolation between the antenna elements forming the array, a relatively large beam-steering angle range with a relatively narrow beam width even at large elevation angles away from said vertical axis, and the capability of synthesizing many different beam polarization states, e.g., linear, right-handed or left-handed, circular, etc. In fact, to maximize the likelihood of detecting the tag, the RFID system may benefit from the flexibility of generating multiple polarization states for each beam steering angle, thus reducing the likelihood that multi-path signal replicas confound a receiver of the reader. This typically requires each antenna element of the array to be more complex, or the design of complex signal-routing networks, both factors being associated with an increased cost and size. In a ceiling-mounted RFID reader, an RFID antenna array can extend away from the ceiling by a distance of as much as 300 millimeters and more. This is undesirably large for a convenient, unobtrusive, aesthetic installation, especially in an existing venue. Although decreasing the distance between the antenna elements and flattening the overall array profile results in a desirably smaller array, it is typically obtained at the expense of lower port isolation and poorer gain and beam-scanning performance caused by mutual coupling between antenna elements and lower gain at large elevations. Mutual coupling between the antenna elements typically results in wasted transmit power during transmission, and a lower received power from incoming signals during reception. It can also limit the effective beam steering angle range.
Accordingly, there is a need for a compact, low profile, low-cost, multi-port, antenna apparatus for an RFID reader with the characteristics of high port isolation, narrow beam width over a large range of steering angles, and high polarization synthesis capability, for enhanced performance.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.